Retrievable sensors are sensors which can be replaced while process fluids are present in the area in which the pressure is to be measured. Such pressure sensors are attached in the pipe wall by way of a flange or bore, for example. It is known to use valve mechanisms for blocking the flow to enable the replacement of the pressure sensor. In practice, the use of valves has proven to be difficult as they are left in an open position and subject to fouling by the process fluid. It is therefore a great risk that the valve is not operable when it is finally needed for the replacement of a pressure sensor, after some 10-15 years, for example. Moreover, when valves are used it is difficult to avoid discharge of process fluid into the environment.
It is therefore a need for a pressure sensor having a reliable structure making it possible to replace pressure sensors in overpressure environments in a simple manner without rendering the pressure sensors inoperable and without having to open up the pipe wall or suspend the production/process. At the same time, the pressure sensor has to meet strict safety requirements.
For underwater installations ROVs (Remotely Operated Vehicles) are used for operating, maintenance and repair and replacement of equipment and sensors on the installation. ROVs are large and may be demanding to manoeuvre in deep ocean depths. This requires positioning of components, equipment and sensors so that an ROV in a simple way may access and perform the operations necessary. For use in underwater installations it is therefore preferable that the pressure sensor may be designed and arranged for easy replacement by an ROV. It may also be envisaged other installations and large processing plants both offshore and onshore where it will be advantageous with a retrievable sensor designed for easy access in connection with maintenance and replacement of the sensor.